Speech typically results in reflected waves. When the reflected wave arrives a very short time after the direct sound, it is perceived as a spectral distortion or reverberation. However, when the reflection arrives a few tens of milliseconds after the direct sound, it is heard as a distinct echo. Such echoes may be annoying, and under extreme conditions can completely disrupt a conversation.
Line echoes (i.e., electrical echoes) occur in telecommunications networks due to impedance mismatches at hybrid transformers that couple two-wire local customer loops to four-wire long-distance trunks. Ideally, the hybrid passes the far-end signal at the four-wire receive port through to the two-wire transmit port without allowing leakage into the four-wire transmit port. However, this would require exact knowledge of the impedance seen at the two-wire ports, which in practice varies widely and can only be estimated. As a result, the leaking signal returns to the far-end talker as an echo. The situation can be further complicated by the presence of two-wire toll switches, allowing intermediate four-two-four wire conversions internal to the network. In telephone connections using satellite links with round-trip delays on the order of 600 ms, line echoes can become particularly disruptive.
Acoustic echoes occur in telecommunications networks due to acoustic coupling between a loudspeaker and a microphone (e.g., in a speakerphone). During a teleconference, where two or more parties are connected by a full-duplex link, an acoustic reflection of the far-end talker through the near-end conference room is returned to the far-end talker as an echo. Acoustic echo cancellation tends to be more difficult than line echo cancellation since the duration of the acoustic path is usually several times longer (100-400 ms) than typical electrical line paths (20 ms), and the acoustic path may change rapidly at any time due to opening doors, moving persons, changing temperatures, etc.
Echo suppressors have been developed to control line echoes in telecommunications networks. Echo suppressors decouple the four-wire transmit port when signal detectors determine that there is a far-end signal at the four-wire receive port without any near-end signal at the two-wire receive port. Echo suppressors, however, are generally ineffective during double-tang when speakers at both ends are talking simultaneously. During double-talk, the four-wire transmit port carries both the near-end signal and the far-end echo signal. Furthermore, echo suppressors tend to produce speech clipping, especially during long delays caused by satellite links.
Echo cancellers have been developed to overcome the shortcomings of echo suppressors. Echo cancellers include an adaptive filter and a subtracter. The adaptive filter attempts to model the echo path. The incoming signal is applied to the adaptive filter which generates a replica signal. The replica signal and the echo signal are applied to the subtracter. The subtracter subtracts the replica signal from the echo signal to produce an error signal. The error signal is fed back to the adaptive filter, which adjusts its filter coefficients (or taps) in order to minimize the error signal. In this manner, the filter coefficients converge toward values that optimize the replica signal in order to cancel (i.e., at least partially offset) the echo signal. Echo cancellers offer the advantage of not disrupting the signal path. Economic considerations place limits on the fineness of sampling times and quantization levels in digital adaptive filters, but technological improvements are relaxing these limits. Echo cancellers were first deployed in the U.S. telephone network in 1979, and currently are virtually ubiquitous in long-distance telephone circuits. See generally Messerschmitt, "Echo Cancellation in Speech and Data Transmission", IEEE Journal on Selected Areas in Communications, Vol. SAC-2, No. 2, March 1984, pp. 283-298; and Tao et al., "A Cascadable VLSI Echo Canceller", IEEE Journal on Selected Areas in Communications, Vol. SAC-2, No. 2, March 1984, pp. 298-303.
In order for the adaptive filter to correctly model the echo path, the output signal of the echo path must originate solely from its input signal. During double-talking, speech at the near-end that acts as uncorrelated noise causes the filter coefficients to diverge (or drift). In open-loop paths, coefficient drift is usually not catastrophic although a brief echo may be heard until convergence is established again. In closed-loop paths (which typically include acoustic echo paths), however, coefficient drift may lead to an unstable system which causes howling and makes convergence difficult. To alleviate this problem, double-talk detectors are commonly used for disabling the adaptation during double-talking. Double-talk detectors may employ, for instance, Geigel's test to detect double-talking. Unfortunately, double-talk detectors fail to indicate the presence of double-talking for a time period (e.g., a whole syllable) after double-talking begins. During this time period, the coefficients may drift and lead to howling as mentioned above. Furthermore, double-talking becomes increasingly difficult to detect as an acoustic echo becomes large in comparison to the near-end signal.
An adaptive echo canceller arranged for overcoming the double-talking problem is proposed in Ochiai et al., "Echo Canceler with Two Echo Path Models", IEEE Transactions On Communications, Vol. COM-25, No. 6, June 1977, pp. 589-595. Ochiai et al. discloses a parallel filter arrangement with a programmable foreground filter and an adaptive background filter. Each of the filters generates a replica of the echo signal. The filter coefficients of the foreground filter are replaced by those of the background filter when the replica signal from the background filter provides a better estimate of the echo signal than the replica signal of the foreground filter. Therefore, during uncorrelated double-talking, the foreground filter is relatively immune from coefficient drift in the background filter. There are, however, drawbacks to this approach. First, in the event the filter coefficients of the background filter drift into an unstable state and are subsequently cleared, there may be a relatively long delay before the background filter works back to providing a better replica signal than the foreground filter. As a result, the convergence time for the foreground filter may be significantly delayed. Secondly, the filter coefficients of the background filter may converge to provide good cancellation for narrowband signals (i.e., a narrow range of frequencies), but poor cancellation outside this frequency range. In this instance, the background filter may provide the foreground filter with an undesirable update.
Based on the foregoing, there is a need for an echo canceller which protects against double-talking, provides rapid convergence, and has robustness to narrowband signals.